The present invention relates to a capacitive distance sensor, in particular, a small-distance (micrometric to millimetric) sensor.
Small-distance sensors are used, among other things, as pressure, proximity, roughness, mechanical stress and acceleration sensors, for example, in integrated microphony and for acquiring fingerprints.
For fingerprint acquisition in particular (to which reference is made herein purely by way of example of a preferred application of the present invention), known sensors include various types, such as optical, piezoelectric, variable-conductance, thermal, ultrasonic and capacitive, the most promising of which in terms of precision, size, production and cost are capacitive sensors.
Capacitive sensors are based on the principle that the capacitance between two capacitor plates is inversely proportional to the distance between them, so that, using the contacting dermal tissue itself as one capacitor plate of the sensor capacitor, and by determining the capacitance, it is possible to locate the ridges and grooves of the fingerprint. This is the principle used in U.S. Pat. No. 5,325,442 to Knapp, which relates to a sensor comprising an array of elementary cells, each comprising a sensitive electrode and an electronic switching device. The electrode is coated with dielectric material, such as passivation oxide or a polymer compound, onto which the finger is placed with the epidermis in contact with the dielectric material. When a cell is selected, a predetermined variation in potential is applied to the electrode to induce at the terminals an appropriate variation in charge. The extent of variation in charge depends on the capacitance associated with the electrode and is read by amplifying elements connected to the output of the device. To improve efficiency, the above patent suggests a surface grid connected to a reference potential to appropriately bias the skin tissue.
In the above known capacitive sensor, the capacitance between the capacitor plates of a capacitor varies in inverse proportion to the distance between the capacitor plates, which therefore poses the problem of normalizing the resulting data. In particular, if the capacitance being measured is very small, serious difficulty is encountered in detecting the charge and discriminating between the various intermediate charge levels corresponding to different grey levels of the image to be generated in the presence of a low signal/noise ratio.
The present invention is directed to a capacitive distance sensor for measuring small distances between the sensor device and an object. The distance sensor includes first and second capacitor plates for forming respective first and second capacitances between the capacitor plates and the object when positioned adjacent to the object. A fringing, third capacitance may also be present between the two capacitor plates. The first and second capacitor plates are arranged in a geometric pattern in which a plurality of faces of the first capacitor plate are positioned adjacent to and facing a plurality of faces of the second capacitor plate. The distance sensor also includes an amplifier having an input connected to one of the capacitor plates and an output connected to another one of the capacitor plates to form a negative feedback branch that includes the first and second capacitances. The sensor device may also include a grounding element surrounding the first and second capacitor plates to provide a discharge path for electrostatic discharge caused by the object.
One embodiment of the invention is directed to a method of making a capacitive distance sensor that includes one or more sensor cells each with first and second capacitor plates. The method includes determining an expected range of sizes of objects the sensor will be used to detect and determining a total perimeter value for each of a plurality of capacitor patterns. Each capacitor pattern includes a different arrangement of the first and second capacitor plates and the total perimeter value is the sum of the perimeter values for the first and second capacitor plates. The method selects one of the capacitor patterns based on the expected size of the object and on the total perimeter values determined for the capacitor patterns. The selecting step includes selecting whichever one of the capacitor patterns has the largest total perimeter value if the object is smaller than each of the one or more sensor cells. The selected capacitor pattern is then formed in at least one of the one or more sensor cells.
Fingerprint and other biometric sensors formed in accordance with the present invention will have application in preventing the unauthorized use of cellular phones, laptop computers, automobiles, automated teller machines, credit/debit card readers, POS terminals, and smart cards. They are also useful in authenticating electronic financial transactions, personal e-mail, providing access to buildings, etc. Biometric identification, such as personal identification and authentication alternatives which could be accomplished by mounting a sensor as taught by the present invention include hand or finger shape, facial shape, and facial features. In addition, the sensor device of the present invention could be used for non-biometric sensing, such as handwriting detection, switch actuation, and any other device requiring sensitivity to object proximity.